1. Field of the Invention
This invention relates broadly to methods for fracturing particles. More particularly, it relates to methods for fracturing particles by impelling them against a hard surface.
2. Problem
A nuclear reactor of the kind designated as a high-temperature, gas-cooled reactor (HTGR) utilizes millions of fuel rods in the form of small cylinders. As disclosed in U.S. Pat. No. 3,901,409 (Bradley et al, issued Aug. 26, 1975) the typical cylinder contains many microspherical particles, including fissile particles. The fissile particles are virtually identical, each consisting of an enriched-uranium oxycarbide microsphere having the following coatings (listed in order from the innermost to outermost): a porous carbon coating, a non-porous isotropic carbon coating, a non-porous silicon carbide (SiC) coating, and another non-porous isotropic carbon coating. The fuel-rod particles are carefully inspected at each stage of their manufacture to determine if they meet specifications with respect to size, shape, density, integrity of the coatings, etc. Particles which fail to meet the specifications are rejected as scrap, and usually it is desirable to process the scrap fissile particles to recover the valuable uranium core, or kernel. A proposed process for such recovery comprises burning off the exterior carbon coating; mechanically fracturing the now-exposed refractory SiC coating; processing the resulting material in a fluidized-bed burner to burn off carbon and convert the uranium oxycarbide to easily leached U.sub.3 O.sub.8 ; and recovering the enriched uranium by acid-leaching.
In connection with the above-described recovery process, an investigation was conducted to find a suitable method and device for mechanically fracturing the SiC coatings. An important specification for the method was that it should fracture the SiC coatings on virtually all of the feed particles, so as to permit recovery by leaching of at least 98 wt-% of the uranium contained in the particle feed. Another important specification was that the fragments from the fracturing operation be sufficiently coarse to be fluidizable in standard fluid-bed burners. A third important specification was that the device should permit long-term use in a "hot cell," where floor space is at a premium and where adjustments and maintenance must be conducted with manipulators. This dictated that the fracturing device should be simple, reliable, compact, and composed of a minimum of moving parts. Preferably, the method would break the SiC coating but minimize breakage of the uranium-containing core of the particles, since fine particles are difficult to fluidize.
Prior Art
A commercially available "centrifugal-impact crusher" includes a spinning disc for slinging particles against anvils positioned outwardly on the rim of the disc. A relatively large (7.5 hp) electric motor rotates the disc at speeds up to 20,000 rpm. As the disc is rotated, particles contained in a central depression in the disc are slung outward through internal radially extending passages. This crusher was considered unsuitable for hot-cell applications because it requires frequent maintenance and is relatively bulky. Various other commercially available crushers (disc pulverizers, fluid-energy mills, etc.) are not suitable for the abovedescribed application because they grind material extremely fine.
A previously developed "jet grinder" was designed especially for breaking SiC-coated particles of the kind described. The grinder includes a column containing a vertical array of air jets and an opposed array of anvils. Particles to be broken are fed to the top of the column and fluidized with air introduced at the bottom. The jets impact the fluidized particles against the anvils. Unfortunately, this device does not provide close control of the inertia of the particles impinged against the anvils. That is, the inertia given to a particle by a jet often is reduced to an unpredictable and significant degree by collisions with other fluidized particles.
A previously developed "roll crusher" was designed to break SiC-coated particles of the kind described. The crusher comprised an electric motor and two gear-driven rollers separated by a fixed spacing. The particles to be broken were fed to the rollers by gravity. The spacing between the rollers could be changed by substituting rollers of a different diameter. That type of crusher is not well adapted for hot-cell applications because the gap between the rollers increases with use and because the mechanism requires frequent maintenance.